This invention relates to the construction of photovoltaic devices and, more particularly, to the construction of a coverglass of a solar cell wherein the coverglass is constructed of an array of thick slabs of glass enclosed by a thin sheet of glass.
Photovoltaic devices are formed of photovoltaic material which interacts with electromagnetic radiation, such as visible light and infrared radiation, by a photoelectric effect wherein incident photons are absorbed and converted to electron-hole pairs. Typically, the photovoltaic devices are formed of layers of different semiconductive photovoltaic material which produce PN junctions at interfaces between the materials. Upon conversion of incident photons to the electron-hole pairs, photovoltages are developed across the PN junctions while photocurrent is supplied to an external load.
Photovoltaic devices are widely used as solar cells in spacecraft for converting solar radiation to electric power used in the performance of spacecraft functions, such as vehicular guidance and communication. Various semiconductor materials having differing bandgap energies may be employed to attain power conversion, from radiant power to electric power, over a band of the solar spectrum; the band may be increased by use of more of the materials. In addition to the generation of electric power, the conversion process also generates heat which is dissipated within the solar cell, and is to be removed from the cell so as to retain an acceptably low operating temperature of the solar cell. It is noted that an excessive rise in temperature of the solar cell reduces the effectiveness and efficiency of the cell operation.
Spacecraft may encounter radiation with high energy particles such as protons and electrons which may impinge upon the solar cell. Such high energy particles interact with the photovoltaic material of the solar cell to alter the conversion characteristics of the semiconductor photovoltaic material with a consequent reduction in the utility of the solar cell. Therefore, it is useful to construct a solar cell with a protective cover, such as a cover glass, which is transparent to the incident radiation, has sufficient mass to protect the cell from high energy particles, and allows for emission of heat.
An attempt has been made to provide an adequate coverglass by employing a thick coverglass fabricated of fused silica or other suitable radiation resistant transparent materials. The silica is made thick enough to inhibit the passage of high energy particles.
A problem arises in that the thick layers of fused silica are far more expensive than desired because such covers must be cut from silica ingots, and must be polished and coated with an ultraviolet (UV) rejection filter plus an antireflective coating. For space travel in regions which do not have the high energy particles found in certain of the Van Allen belts, a thinner coverglass made of ceria-doped sodium line glass can be employed to provide protection from weaker atomic particles. The ceria-doped sodium lime glass should not be formed as a thick glass element because such thick elements have higher absorption of radiant energy than fused silica, and cannot be used without a large transmittance penalty.